Mullins mostly played right offensive guard for the Pittsburgh Steelers for his entire professional career (1971–1979), next to centers Ray Mansfield (1971–1975) and Mike Webster (1976–1979). Known as "Moon," Mullins was drafted in the fourth round of the 1971 NFL Draft and went on to win four Super Bowls with the Steelers (Super BowlsIX, X, XIII, and XIV). He can be remembered for a play in Super Bowl IX when he pulled on a sweep leaving an uncontested path for Franco Harris to trot in the end zone. Mullins also recovered an onside kick in Super Bowl X. Mullins retired after Super Bowl XIV.